A vehicle may be equipped with a distance measurement device that measures an inter-vehicle distance between a subject vehicle and a lead vehicle in front of it. Such a distance measurement device may emit or transmit, a laser beam toward a front field of the subject vehicle and receives a reflection of the emitted beam reflected by the lead vehicle. The device, a so-called laser radar or an optical distance measurement sensor, disclosed in a patent document, Japanese Patent Laid-Open No. 2013-8302 (Patent document 1) is used to detect an inter-vehicle distance. The laser radar or the optical distance measurement sensor has a high measurement accuracy.
The optical distance measurement sensor uses a light beam for the distance measurement, and the light beam has a high straight-transmission stability of running straight, or traveling forward. Therefore, in situations where the lead vehicle climbs uphill while the subject vehicle is still traveling a flat part of the road, or where the lead vehicle is already traveling the flat part of the road while the subject vehicle is still traveling downhill, the laser beam may be reflected by the road surface instead of being reflected by the lead vehicle. That is, the road surface may erroneously be detected as the lead vehicle by the laser radar in the above-described situations.
Further, the electric wave has a high straight-transmission stability at high frequencies. Therefore, the electro-magnetic distance measurement sensor may also erroneously detect the road surface as the lead vehicle, when the detection wave is in high frequencies. Even with the low frequency wave, the road surface may still be erroneously detected as the lead vehicle, with the possibility of such error detection decreasing just a little bit down from the high frequency waves.
The erroneous detection of the road surface as the lead vehicle leads to an erroneous control of an automatic function based on the inter-vehicle distance to the lead vehicle, such as an automatic braking, an automatic steering and the like.